Cannot invoke compareTo(double) on the primitive t

问题:

The line return array[index1].compareTo(array[index2]); provides an error "Cannot invoke compareTo(double) on the primitive type double". How to solve this issue?

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function implements a comparator of double values        :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
private class ArrayIndexComparator implements Comparator<Integer>
{
    private final double[] array;

    public ArrayIndexComparator(double[] array)
    {
        this.array = array;
    }

    public Integer[] createIndexArray()
    {
        Integer[] indexes = new Integer[array.length];
        for (int i = 0; i < array.length; i++)
        {
            indexes[i] = i; // Autoboxing
        }
        return indexes;
    }

    @Override
    public int compare(Integer index1, Integer index2)
    {
         // Autounbox from Integer to int to use as array indexes
        return array[index1].compareTo(array[index2]);
    }
}


double[] dist = new double[centroids.size()];
// fill array...    
ArrayIndexComparator comparator = new ArrayIndexComparator(dist);
Integer[] indexes = comparator.createIndexArray();
Arrays.sort(indexes, comparator);

回答1:

Replace the call of an instance method compareTo with the call of static compare method, like this:

return Double.compare(array[index1], array[index2]);

This lets you keep your doubles in an array of primitives, and avoid autoboxing before calling an instance method.

回答2:

for primitive types do not use compareTo, use == instead

but if you want to use compareTo just create a Double array

Double[] dist = new Double[centroids.size()];

回答3:

In java primitive types don't have any methods. Instead using primitive data types use Wrapper classes.

change

return array[index1].compareTo(array[index2]);

to

return  new Double(array[index1]).compareTo(array[index2]);

or

try with Double[] array; instead of double[] array;

回答4:

Primitive Types cannot be compared directly by a comparator, as the interface is only implemented by collator and RuleBasedCollator. No wrapper class implements comparator. Due to which compiler won't be able to auto box it.

Just look in Double class and you will find an inbuilt method which provides compare method.

public static int compare(double d1, double d2)

Returns: the value 0 if d1 is numerically equal to d2; a value less than 0 if d1 is numerically less than d2; and a value greater than 0 if d1 is numerically greater than d2.

Reverse: Multiple the entire expression by -1;